Applying energy to a mobile device is mainly done with a charger or an external power supply. Purpose of it is to supply the system with power and charge and/or maintain the battery.
The fact that many users carry one or more spare batteries with them is widely spread. Especially in the developing countries, where a mobile phone call is one of the most important use cases of the mobile device, battery ma runs out of energy quite fast. Users exchange an empty with a full charged battery simply because often the infrastructure for simple recharging is missing (no power grid, no mains).
Current problem with existing designs is that a nearly empty battery which is planned to be removed and replaced with a fully charged one forces the system to be shut down and the reboot once the fully charged battery is inserted and that the mobile terminal is switched on.
In some systems the start up and fully reboot of the system may take a couple of minutes, dependent on the complexity and usage of the mobile terminal.